What are the different options for excluding certain post types from search results?

You can use the pre_get_posts hook. Do note though, this hook filters the query on both the front-end and admin. For example, we could do something like this:

function search_post_types( $query ) {
    if( is_admin() ) {
        return $query;
    }

    if ( $query->is_search && $query->is_main_query() ) {
        $query->set( 'post_type', array( 'post', 'page' ) );
    }
}
add_action( 'pre_get_posts', 'search_post_types' );

I’m not sure there’s a way to exclude it using pre_get_posts but more along the lines of include specific post types only, in the example above I’m only including Posts and Pages, nothing else.

---

Another method is to get all post types and create an array of post types we actually want to exclude. First we use the function get_post_types() which does have some arguments to exclude built-in post types but for this example we will get everything. Once we’ve gotten our post types we can create an exclude array and array_diff(), here’s what it looks like:

function search_post_types( $query ) {
    if( is_admin() ) {
        return $query;
    }

    if ( $query->is_search && $query->is_main_query() ) {
        $post_types = get_post_types( array(), 'names' );                   // With no arguments, this should never be empty
        if( ! empty( $post_types ) ) {                                      // But let's check just to be safe!
            $pt_exclude = array( 'attachment', 'revision', 'nav_menu_item' );
            $pt_include = array_diff( $post_types, $pt_exclude );

            $query->set( 'post_type', $pt_include );
        }
    }
}
add_action( 'pre_get_posts', 'search_post_types' );